TRIASSIC FISHES FROM SPIl'ZBERGEN 
187 
the latter bone-bed present a partially different appearance (PI. 21, fig. 7). These deviating 
teeth have the enamel-covered part basally of the enamel cap developed with fairly coarse 
vertical ridges, and these ridges have then in their turn fine striae or fine folds in the 
longitudinal direction. There are, however, several transitional forms between this type 
of tooth and the others. 
Of special interest are the teeth of the branchial arches that are preserved in situ 
on one specimen from the fish horizon (P. iji). These are of about the same height as 
those on the lateral part of the ectopterygoid and the dorsal edge of the mixicoronoid. 
Relatively to their length they are, however, more slender than other teeth and also have 
a longer and more pointed enamel cap than these. Their enamel caps is also characterized 
by the fact that, unlike that in the other teeth, is seems to be without striation. 
We thus see that the teeth of the branchial arches differ considerably from the 
other teeth in the same species, and the difference is even so great that we should 
probably not have ventured to refer them to the same species as the others if they 
had not been found in the same individual as these. We thus get an idea of the diffi¬ 
culty of distinguishing different fish species from each other merely by means of 
detached teeth. 
Nothing certain is known as to the arrangement of the teeth on the branchial 
arches. 
All the teeth of the fish in question have a large pulp cavity that extends up 
into the enamel cap. This cavity is surrounded mostly by a layer of ortho-dentine, 
which is rather thin towards the base a fact that explains why detached teeth of 
Birgeria species practically never have their basal parts preserved. Outside the layer of 
ortho-dentine there follows in the distal part of the teeth the enamel cap, and this is 
continued basally, as I have mentioned above, by the thin enamel covering which seems 
to stop at the boundary between the middle and basal thirds of the teeth. At any rate 
it does not reach as far down as to the base. This enamel covering seems to have 
been overlooked by Agassiz (1844, vol. II, part II, p. 153). 
The Birgeria teeth show, as we know, a great resemblance to the Saurichthys teeth, 
which has also caused B. mougeoti to be adopted as a Saurichthys species. We shall 
see, however (cf. part II), that a very characteristic quality of Saurichthys teeth is that 
they have the dentine somewhat folded (plicidentine) in their basal parts, which is not 
the case in Birgeria, and in addition that their enamel cap generally lacks striation. If 
striation is developed it is, however, extremely delicate and abundant. 
Axial skeleton. 
The axial skeleton in B. mougeoti is represented by a number of remains from the 
caudal region and the most posterior part of the abdomial region. The remains from 
the latter region are very incomplete and little suited for a more thorough investigation. 
I have therefore been compelled to confine myself almost entirely to a description of 
the axial skeleton in the caudal region. Where nothing else is stated the description 
is based on specimen P. 155. 
It is evident that the chorda dorsalis has played an important part and has been 
well developed. A thin brownish layer (the brown colour due to the infiltration of 
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